1. Field of the Invention
The present invention relates to a method of driving a piezoelectric actuator and to a method of driving a liquid ejection head, and more particularly, to technology for driving an orientated piezoelectric element (orientated piezoelectric actuator) which is deposited by a technique such as sputtering.
2. Description of the Related Art
An inkjet recording apparatus which forms a desired image by ejecting ink droplets from an inkjet head onto a recording medium is widely used as a generic image forming apparatus. In an inkjet recording apparatus, piezoelectric elements (piezoelectric actuators) are suitable for use as pressure application devices which cause ink droplets to be ejected from the inkjet head.
Improved printing performance, and in particular, higher image resolution and faster printing speed, are demanded in inkjet heads. Consequently, it has been attempted to increase the image resolution and to raise the printing speed, by using a multiple-nozzle head structure in which the nozzles are formed to a very fine size and are arranged at high density. In order to achieve a high-density arrangement of nozzles, it is highly important to achieve a compact size of the piezoelectric elements which are pressure generating elements.
In order to form the piezoelectric elements to a compact size, it is effective to reduce the thickness of the piezoelectric elements, and for example, Japanese Patent Application Publication No. 10-286953 discloses technology for forming a lead dielectric layer (piezoelectric film) having a film thickness of 3 μm by sputtering, in order to achieve a thin film thickness in the piezoelectric elements.
It is possible to obtain a similar amount of displacement of a piezoelectric element (piezoelectric actuator) which is generally used in an inkjet head irrespectively of the direction of the electric field applied. The piezoelectric element (piezoelectric actuator) 358 for the inkjet head 350 illustrated in FIG. 20 uses an upper electrode 357B as an address electrode, and a lower electrode (substrate surface) 357A as a ground electrode, and is driven by applying a positive electric field to the address electrode side (an electric field in the electric field direction indicated by an arrow in FIG. 20). Reasons for adopting an electrode structure of this kind relate to the cost of the driving IC (driver circuit) and other components, and the ease of wiring, and the like.
However, the piezoelectric film (PZT film) manufactured by sputtering which is described in Japanese Patent Application Publication No. 10-286953 has a direction of orientation (polarization) (mainly, (100), (001), (111)) which is determined when the film is deposited, and therefore produces a different amount of displacement depending on the direction of the electric field applied. FIG. 21 illustrates a relationship of the amount of displacement of a piezoelectric actuator comprising a piezoelectric film having the (100) orientation, with respect to the applied electric field.
If the piezoelectric film 358A (piezoelectric element 358) in FIG. 20 is manufactured by sputtering, then the film is polarized in the direction from the lower electrode 357A toward the upper electrode 357B during the deposition of the film, and therefore in order to make the diaphragm 356 deform toward the lower side in FIG. 20 (in order to obtain displacement in the positive direction indicated by an arrow in FIG. 20), an electric field must be applied in the direction from the lower electrode 357A toward the upper electrode 357B (an electric field in the opposite direction to the electric field direction indicated by the arrow in FIG. 20).
When an electric field is applied in this direction, the upper electrode 357B is taken as an address electrode, the lower electrode 357A is taken as a ground electrode and a negative electric field must be applied to the upper electrode 357B, which means that the costs of the drive IC (driver circuit) and power supply are several times to several tens of times greater than when applying a positive electric field to the upper electrode 357B.
Furthermore, from the viewpoint of reducing costs, when the piezoelectric element 358 is driven by applying a positive electric field to the lower electrode 357A, if the diaphragm 356 is made of silicon, then there may be a problem of electrical cross-talk in which a leak current 360 occurs between mutually adjacent lower electrodes as illustrated in FIG. 22, and the diaphragm is displaced even at piezoelectric elements which are not driven and to which an electric field is not applied, and in a worst case scenario, ink is ejected from pressure chambers (nozzles) where it is not supposed to be ejected. Moreover, due to the increase in the electrostatic capacitance, there is also a drawback in that the power consumption increases.
As a method for avoiding problems of this kind, there is a method of manufacturing an inkjet head by sequentially depositing, by sputtering, an upper electrode, a piezoelectric film and a lower electrode onto a monocrystalline substrate made of silicon (Si), magnesium oxide (MgO), or the like, (a so-called “dummy substrate”), thereby fabricating a piezoelectric element structure (piezoelectric actuator) having a thin film which is to form a diaphragm on top of the lower electrode, whereupon the piezoelectric element structure is inverted mechanically and transferred (bonded) to a pressure chamber formed in a silicon substrate or a glass substrate (for example, the silicon base material having a pressure chamber 352 formed therein in FIG. 20).
However, in a method in which a previously manufactured piezoelectric element structure is reversed mechanically and transferred to a pressure chamber, there is a tendency to raise costs because a monocrystalline substrate is thrown away after use. Moreover, it is necessary to align the piezoelectric element structure and the pressure chamber accurately in order to use a transfer bonding method, and accurate positional alignment between the piezoelectric actuator and the pressure chamber is extremely difficult to achieve. The accuracy of positional alignment of the piezoelectric actuator and the pressure chamber affects the ejection characteristics, and in an inkjet head comprising a plurality of nozzles, it is extremely difficult to fabricate a head having uniform ejection characteristics in each of the nozzles.
To summarize the problems relating to an oriented piezoelectric film as described above (for example, a piezoelectric film deposited by sputtering), a method which mechanically reverses a piezoelectric element and bonds same to a pressure chamber involves the problem of positional alignment accuracy during bonding, and a method which uses a lower electrode 110 as an address electrode involves the problem of electrical cross-talk occurring as a result of leakage current. Furthermore, a method which uses an electric field in the negative direction as the applied electric field involves the problem of increased costs in relation to the IC, and so on (see Table 1 below).
TABLE 1IssueMethod of resolutionProblemOppositeTransfer previously manufacturedPositional alignmentdirectionpiezoelectric element to pressureis difficultofchamberorientationLower address structureLeakage currentNegative drivingHigh costs